The Circular Ring Sector tool lets you create a pie sliced portion of a Circular Ring like this:
You select the center first and then drag outward to define the first radius of the sector and mark it with the Mark Button.
Unlike the Circular Ring Sector tool, the only Snapping Points on a Circular Ring Sector are the four corners of the sector and the center point of sector. This is because the sector is actually just an approximation of a circular sector using a Rectangle and well placed Bezier Points. This means that you can actually operate on a Circular Ring Sector using the Move Bezier Point tool. This isn’t possible on a Circular Ring.